1. Field of the Invention
The present invention relates to a process for the preparation of finely divided, porous and rapidly water-swellable polysaccharide graft polymers. These polymers are prepared by semicontinuous reverse-phase suspension polymerization, partial dewatering and subsequent cross-linking.
2. Description of the Background
Water-absorbent polymers are widely used in the sanitary and hygiene sectors as water absorbents in paper napkins and cloths, as tampons, patient undersheets, electrolyte thickeners in dry batteries, as humectants and water-storage agents in agriculture and as desiccants. Suitable polymers include polysaccharides, usually grafted with water-soluble vinyl monomers, such as carboxymethylcellulose, hydrolysed starch-acrylonitrile graft polymers, acrylic acid-starch graft polymers, and fully synthetic, weakly crosslinked polymers, such as partially crosslinked polyacrylic acid salts and partially crosslinked polymaleic acid derivatives.
The incorporation of starch into water-swellable graft polymers allows particular product properties to be produced, compared with fully synthetic polymers. Thus, the porosity of the polymer particles is increased, the absorption rate is increased and the biodegradability is improved.
The preparation of the graft polymers by direct grafting of starch with acrylate in aqueous solution is not technically simple. In order to achieve the most homogeneous distribution possible, which is necessary for the grafting, of starch in the aqueous monomer solution, prior swelling of the starch is necessary. This considerably increases the viscosity of the monomer solution, a paste-like consistency being obtained if more than about 10% of starch is used.
DE-C 26 12 846 discloses the preparation of water-absorbent graft polymers by grafting water-soluble monomers, such as acrylic acid, onto starch in the presence of a crosslinking agent. The grafting reaction is carried out in aqueous solution or in an aqueous-alcoholic medium as a precipitation polymerization. The grafting in aqueous solution in this process gives rubber-like, non-stirrable gels from which pulverulent end products can only be obtained after drying and grinding. By contrast, if the grafting is carried out as a precipitation polymerization in the presence of an approximately 20-fold excess of alcohol as precipitant, finely divided products having an only moderate liquid-absorption capacity are produced.
According to Japanese Patent 80/139,408, polymerization of acrylonitrile in aqueous medium in the presence of starch results in the preparation of a graft polymer, which can subsequently be hydrolysed and crosslinked. A powder having a water-absorption capacity of from 150 to 180 ml/g is obtained.
Finely divided water-absorbent polymers can be prepared by polymerization of partially neutralized acrylic acid in inverse suspension, also known as reverse-phase suspension. In this process, a water-soluble monomer in the form of an aqueous solution is emulsified in a hydrophobic medium in the presence of a dispersant and polymerized to give a finely divided end product. In the presence of a polysaccharide, such as starch, water-soluble or water-insoluble, swellable graft polymers can be obtained.
A graft polymerization in inverse suspension is described in Japanese Patent 80/161,813. In this process, a mixture of n-hexane and sorbitan monostearate, starch, water, acrylic acid, sodium hydroxide solution and water-soluble initiator is first prepared before the polymerization is initiated by warming. However, the reaction product tends to form lumps during the polymerization and is not produced in finely divided form.
DE-C-28 40 010 discloses the preparation of water-swellable polysaccharide graft polymers by inverse suspension polymerization, a batch process being preferred. In this process, a polysaccharide is first suspended in a water-immiscible solvent in the presence of a surface-active agent. An aqueous monomer solution, predominantly containing acrylamide or a cationic monomer and possibly also small amounts of acrylic acid, is subsequently added at room temperature. After addition of an initiator, the mixture is warmed and polymerized. The solid contents, based on the aqueous polymerization mixture, are above 50%. This process does not use any crosslinking agents and does not produce any water-swellable, gel-like polymers. The batchwise preparation process results in temperature peaks at the beginning of the polymerization which are difficult to control in large batches.
In EP-B-0 083 022, acrylic acid is polymerized in the presence of starch in aqueous solution. The products can then be crosslinked in an inert solvent in the presence of from 0.01 to 1.3 parts of water per part of resin. The preparation of the starch graft polymers is carried out at a solids content of only 20%. In addition, these starch graft polymers have only a low water-absorption capacity, whether crosslinked or uncrosslinked.
According to DE-A-38 01 633, polysaccharide graft polymers are prepared by inverse suspension polymerization, partial dewatering and crosslinking. However, the inverse suspension polymerization is carried out in one step and batchwise by first combining all the reaction components and then initiating the polymerization by heating in the presence of an initiator. The beginning of the polymerization is accompanied by the sudden liberation of heat. In industrial production reactors, sufficiently rapid dissipation of the heat of polymerization can frequently only be ensured with difficulty.
In German Patent Application P 40 14 628.6, the preparation of polysaccharide graft polymers is further improved. Here, a semicontinuous reverse-phase suspension polymerization is carried out in which an aqueous solution of an unsaturated carboxylic acid and an initiator is added continuously to a suspension of a polysaccharide in an organic solvent containing a dispersant combination. This process allows reliable dissipation of the heat of polymerization. However, an increase in viscosity, which is associated with increased energy consumption by the stirrer, follows the completion of the feed of the aqueous solution and accompanies the beginning of the subsequent azeotropic partial dewatering. The high viscosity increases the tendency of the swollen polymer particles to agglomerate. A need therefore continues to exist for an improved procedure for the preparation of polysaccharide graft polymers by reverse phase suspension polymerizatin which prevents viscosity increases of the polymerization medium.